1. Field of the Invention
This invention relates to a switch for a variable-speed induction motor, particularly to a switch for a variable-speed induction motor of an electrical fan.
2. Description of the Related Art
Variable-speed induction motors for electrical devices, such as electric fans, are normally classified into two types, namely capacitor motor and tapped winding motor. A capacitor motor is generally noiseless upon varying the speed thereof. However, it is expensive, and tends to generate a large amount of heat during the operation of the motor. A tapped winding motor normally includes an electrical circuit board having a plurality of spaced apart conductive traces connected respectively to a plurality of coils, and a turnable rotary member with a plurality of spaced apart conductive connecting pieces which selectively bridge to the conductive traces by turning the rotary member. The rotation of the rotary member varies the points of contact of the conductive connecting pieces on the conductive traces of the electrical circuit board, thereby resulting in a plurality of possible combinations of electrical connections of the electrical circuit board with the coils for varying the speed of the motor. However, there is a tendency to generate noise due to a non-uniform magnetic line distribution formed between the charged coils upon varying the speed of the motor. Moreover, an electrical arc is not only generated between the conductive connecting pieces and the conductive traces at the very beginning of the rotation of the rotary member, but also during the course of the rotation of the rotary member.
Therefore, the object of the present invention is to provide a switch for a variable-speed induction motor that is capable of overcoming the aforementioned problems.
According to the present invention, a switch for a variable-speed induction motor comprises: a casing having a pair of spaced apart first conductive connecting pieces, one of the first conductive connecting pieces being adapted to be connected to a power supply; a rotary member disposed below and mounted rotatably on the casing for turning about an axis, and having a toothed part, and a plurality of spaced apart second conductive connecting pieces secured to a bottom side thereof; a switching electrical circuit adapted to be connected to the induction motor, disposed below the second conductive connecting pieces, and having a plurality of spaced apart conductive traces connected to the other one of the first conductive connecting pieces, the second conductive connecting pieces selectively bridging the conductive traces by turning the rotary member about the axis to obtain a plurality of combinations of electrical connections for the switching electrical circuit to vary the speed of the induction motor; a driving member mounted movably on the casing, and having a third conductive connecting piece projecting toward the first conductive connecting pieces, and a tongue projecting toward the toothed part, the driving member being movable between a first position, in which the third conductive connecting piece interconnects the first conductive connecting pieces to permit the switching electrical circuit to be connected electrically to the power supply, and a second position, in which the third conductive connecting piece disconnects the first conductive connecting pieces, thereby disconnecting the switching electrical circuit from the power supply, the tongue engaging the toothed part and turning the rotary member a pitch of the toothed part each time the driving member moves from the first position to the second position; and an urging unit for urging the driving member to move from the second position to the first position.